Various processes of purification of aromatic dicarboxylic acids such as isophthalic acid and terephthalic acid are known in the prior art. U.S. Pat. No. 2,664,440, issued to Toland, discloses a process for separating isophthalic and terephthalic acids from the mixtures by dissolving the acids in a tertiary amine, crystallizing the dissolved salts, and then decomposing the salts to their respective acids. U.S. Pat. No. 2,881,548, issued to Ham et al, discloses a process for purifying aromatic dicarboxylic acids using a recrystallization solvent. The recrystallization solvent can be N,N-dimethylformamide, N,N-dimethylacetamide, mixtures of N,N-dimethylformamide and water, N,N-dimethylacetamide and water, N-N-dimethylformamide and methanol, and N,N-dimethylacetamide and methanol. U.S. Pat. No. 3,849,489, issued to Rudzki, discloses a process for purifying terephthalic acid by forming an ammonium terephthalate solution by reacting terephthalic acid in ammonia water, separating the ammonium terephthalate from the solution, and decomposing the ammonium terephthalate so as to produce terephthalic acid. The terephthalic acid is redissolved in ammonia water, the impurities in solution oxidized and the solution then filtered and contacted with activated charcoal. Ammonium terephthalate is separated from the process solution and decomposed to produce terephthalic acid. The formation of diphenylisophthalate/diphenylterephthalate monomer and other similar monomers can take place by various methods known in the prior art. U.S. Pat. No. 3,068,206, issued to Nicolson et al, discloses a polyester synthetic reaction system wherein one or more polyhydric alcohols such as ethylene or propylene glycol or glycerol are reacted with one or more polycarboxylic acids or their anhydrides. A modifying agent may or may not be used. The reaction is preferably carried out at temperatures from room temperaure to about 300.degree. C. U.S. Pat. No. 3,039,980, issued to Mallison, discloses a polyhydric alcohol/polycarboxylic acid reaction wherein water of the esterification reaction is removed from the reaction zone by passing through the reaction zone an inert, organic, normally liquid solvent. U.S. Pat. No. 3,109,831, issued to Seiner, discloses reaction between a polyol and polyester of a dicarboxylic acid whereby the volatilization of the polyol is held to a minimum. U.S. Pat. No. 3,819,585, issued to Funk et al, discloses the formation of a polyester from ethylene glycol and terephthalic acid. The reaction has two phases wherein the first phase is conducted at a pressure of about 0-5 psig through about 74-95% of esterification, and the second phase is conducted at a substantially higher pressure than the first phase during which the percent of esterification is increased to about 95-99%. U.S. Pat. No. 4,146,729, issued to Goodley et al, discloses an esterification reaction between terephthalic acid and ethylene glycol whereby the excess glycol is recovered from reaction off-gases. U.S. Pat. No. 4,204,070, issued to Suzuki et al, discloses a process for preparing bis(hydroxyethyl)terephthalate where terephthalic acid is dissolved in molten bis(hydroxyethyl)terephthalate and its low molecular weight oligomers at a temperature above the dew point of ethylene glycol. Ethylene glycol is fed into the solution at a controlled rate so that the ethylene glycol does not distill off. U.S. Pat. No. 4,254,246, issued to Dicoi et al, discloses a reaction of ethylene glycol and terephthalic acid in the present of a catalyst so that the vapor by-products from the esterification, prepolycondensation and polycondensation stages are continuously rectified yielding high-purity ethylene glycol (99.85%) which is returned to the process.
U.S. Pat. No. 3,418,286, issued to Schmidt et al, discloses the reaction of a polyhydric alcohol and a carboxylic acid to produce a polyester resin by a fusion process. The reaction can be carried out at temperatures above 350.degree. F. A vacuum or reduced pressure is also applied to the system, but not until the reaction has gone 1/3 of the way to completion and before the reaction has gone 3/4 of the way to completion.